Laser polishing of a back contact of a solar cell

ABSTRACT

Provided herein are methods of polishing, cleaning and texturing back contacts of thin-film solar cells. According to various embodiments, the methods involve irradiating sites on the back contact with laser beams to remove contaminants and/or smooth the surface of the back contact. The back contact, e.g., a molybdenum, copper, or niobium thin-film, is smoothed prior to deposition of the absorber and other thin-films of the photovoltaic stack. In certain embodiments, laser polishing of the back contact is used to enhance the diffusion barrier characteristics of the back contact layer, with all or a surface layer of the back contact becoming essentially amorphous. In certain embodiments, the adhesion of the absorber layer is enhanced by the textured back contact and by the presence of the amorphous metal at the deposition surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/357,277, filed Jan. 21, 2009, which claims benefit of thefollowing: U.S. Provisional Patent Application No. 61/022,232, filedJan. 18, 2008 and U.S. Provisional Patent Application No. 61/022,240,filed Jan. 18, 2008. This application is also a continuation-in-part ofU.S. patent application Ser. No. 12/049,159, filed Mar. 14, 2008. Eachof these applications is incorporated by reference herein for allpurposes.

BACKGROUND

Solar cells are widely used to collect solar energy and generateelectricity, with multiple solar cells interconnected to form solarpanels. There are two main types of solar collectors: silicon andthin-film solar collectors. Silicon is currently the predominanttechnology, using silicon-based cells encapsulated behind glass.Thin-film processes involve depositing thin-film materials on asubstrate to form a light absorbing layer sandwiched between electricalcontact layers. The front or top contact is a transparent and conductivelayer for current collection and light enhancement, the light absorbinglayer is a semiconductor material, and the back contact is a conductivelayer to provide electrical current throughout the cell.

In one example of a fabrication process, a metallic back electricalcontact layer is deposited on a substrate. A p-type semiconductor layeris then deposited on the back electrical contact layer and an n-typesemiconductor layer is deposited on the p-type semiconductor layer tocomplete a p-n junction. Any suitable semiconductor materials, such asCIGS, CIS, CdTe, CdS, ZnS, ZnO, amorphous silicon, polycrystallinesilicon, etc. may be used for these layers. A top transparent electrodelayer is then deposited on the p-n junction. This layer may be aconductive oxide or other conductive film and is used for currentcollection. Once these or other materials have been deposited on thesubstrate to form a photovoltaic stack, the substrate and thin-filmmaterials deposited may be cut into cells. Multiple cells are thenassembled into a thin-film solar panel.

Thin-film solar collectors are lower cost and require lower energy toproduce. However, there are numerous sources of defects in a thin-filmpanel, given the materials used, the large surfaces areas required, thesmall thicknesses of films, and the sensitivity of the films to smallparticles. For example, a one micron particle is on the order of thefilm thickness. Defects from particles and other sources can causeshunts and otherwise compromise solar cell performance.

SUMMARY

Provided herein are methods of polishing, cleaning and texturing backcontacts of thin-film solar cells. According to various embodiments, themethods involve irradiating sites on the back contact with laser beamsto remove contaminants and/or smooth the surface of the back contact.The back contact, e.g., a molybdenum, copper, or niobium thin-film, issmoothed prior to deposition of the absorber and other thin-films of thephotovoltaic stack. In certain embodiments, laser polishing of the backcontact is used to enhance the diffusion barrier characteristics of theback contact layer, with all or a surface layer of the back contactbecoming essentially amorphous. In certain embodiments, the adhesion ofthe absorber layer is enhanced by the textured back contact and by thepresence of the amorphous metal at the deposition surface.

One aspect of the invention relates to a method of polishing a backcontact of a thin-film photovoltaic stack. The method involves providinga substrate having a back contact layer for a thin-film photovoltaicstack deposited thereon; and polishing a front surface of the backcontact layer by forming surface features in the front surface, whereinforming surface features comprises rastering one or more laser beamsover the front surface. The back contact layer may be molybdenum,niobium, copper and silver or any other thin-film capable of makingohmic contact. The substrate may be a metal substrate, e.g., a stainlesssteel foil.

In certain embodiments, the method involves depositing one or morethin-film layers configured to absorb solar energy on the polished backcontact. Examples of absorber layers include CIGS or CIS. In certainembodiments, rastering one or more laser beams over the front surfaceincreases the amorphous character of the back contact layer, and mayinclude improving the diffusion barrier characteristics of the backcontact and/or increasing adhesion of the absorber layer to the backcontact.

In certain embodiments, the substrate is providing as a web. Polishing afront surface of the back contact layer may involve forming multiplerows of surface features in the front surface, wherein forming a row ofsurface features comprises rastering one or more laser beams over thefront surface in a cross-web direction. The surface features of the rowmay overlap with the surface features of each adjacent row.

In certain embodiments, the back contact is about 0.5 microns thick. Inalternate embodiments, the back contact is between about 1 and 5 micronsthick. In certain embodiments, the back contact thickness is at leastabout 5 microns to allow texturing and polishing depths of this size.According to various embodiments, the substrate may or may not bepolished by laser polishing or other techniques prior to deposition ofthe back contact.

In certain embodiments, the polishing or texturing methods describedherein involve irradiating a front surface of the back contact layerwith laser energy to heat the surface in localized areas of between 5 to200 microns and form a plurality of shallow, rapidly expanding meltpools; and after forming the melt pools, cooling the front surface toform a smoothed surface. Irradiating the front surface with laser energymay involve exposing the front surface to laser beams at a rate of about10-1000 kHz. The smoothed surface may be characterized by a pattern ofinterconnected surface features, said features having a frequency of1-200 microns. The interconnected surface features may have an averagemaximum aspect ratio of no more than 1:1, in certain embodiments,substantially smaller than 1:1, e.g., 0.5:1, etc. Also in certainembodiments, the substrate is oriented at an angle of no more than 10°from vertical during the irradiation.

These and other aspects of the invention are discussed in further detailbelow with reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the embodiments of theinvention:

FIG. 1 depicts schematic illustrations of a thin-film solar cell and twomonolithically integrated thin-film solar cells according to certainembodiments.

FIG. 2A depicts a schematic illustration of laser polishing of asubstrate of a thin-film photovoltaic stack according to certainembodiments.

FIG. 2B depicts a schematic illustration of laser polishing of a backcontact of a thin-film photovoltaic stack according to certainembodiments.

FIG. 2C depicts a schematic illustration of laser cleaning of a surfaceof a layer in a thin-film photovoltaic stack according to certainembodiments.

FIG. 3 is a simplified diagram showing a deposition apparatus fordepositing thin film photovoltaic materials on a web substrate includinga laser polishing apparatus according to certain embodiments.

FIG. 4 depicts schematic illustrations of examples of high frequencyroughness, low frequency roughness (waviness) and flatness scales ofthin-film photovoltaic stacks according to certain embodiments.

FIG. 5 depicts top and side views of an overlapping texture patternaccording to certain embodiments.

FIG. 6 is an image showing an example of a laser textured feature formedin a nickel surface.

FIG. 7A shows two images of surface finishes of 300 series stainlesssteel surface prior to laser treatment.

FIG. 7B shows an image of a surface finish of a 300 series stainlesssteel surface after laser treatment according to certain embodiments.

FIG. 8 shows two examples of texturing a surface of substrate in anellipsoidal pattern according to certain embodiments.

FIG. 9 is a process flow diagram illustrating operations in a method oflaser polishing or texturing a back contact film of a thin-filmphotovoltaic stack according to certain embodiments.

FIGS. 10 and 11 are process flow diagrams illustrating operations inmethods of incorporating sodium into a thin-film photovoltaic stackaccording to certain embodiments.

FIGS. 12A and 12B depict components of a laser polishing apparatusaccording to certain embodiments.

FIGS. 13A and 13B depict components of a laser treatment apparatusaccording to certain embodiments.

DESCRIPTION OF EMBODIMENTS

Thin-film solar panels have many advantages, including providing lowcost, low energy production. However, there are numerous sources ofdefects in a thin-film panel, given materials used, the large surfacesareas required, the small thicknesses of films, and the sensitivity ofthe films to small particles. For example, a one micron particle is onthe order of the film thickness. Defects from particles and other causescan cause shunts and otherwise compromise solar cell performance.

Another factor related to defects is the level of substrate preparation.For semiconductor device fabrication, substrate (semiconductor wafer)preparation involves chemical-mechanical planarization and cleaningprior to wafers being released into the line. In media production, nearsub-single nanometer polish levels are achieved. However, thesetechniques are highly complex and expensive in comparison to what canreasonably be done as pre-preparation prior to film deposition for alarge area solar panel. Any large area, economical method of substratepreparation is likely to leave some level of high-frequency roughness,in comparison to the film thickness. As a result, any robust and highyielding thin-film solar product has a certain number of defectsaccounted for in the product and process design. Unlike wafer flatnessrequirements for photolithography, probing and other concerns, andmagnetic media for spacing control, there are no significant waviness orflatness requirements for large area photovoltaic panels.

However, defects and particles should be minimized as they can causeshunts between the top and back contacts of a photovoltaic stack. Shuntsites may be present on fabrication or can develop during the lifetimeof a solar panel. This development can be related to moisture ingress,collection of ionic contaminants, changes in stress/strain in the filmor package or thermal cycling. A new site near a collection line cansink nearly all current generated in a typical cell, and can actuallymelt the encapsulating material, allowing for additional environmentaldegradation. The use of serialized cell strings where the voltages areincreased across an individual cell place even greater stresses ondefect sites and film continuity, the opportunity for field damage tooccur in a shaded or poor performing cell. In addition to shunt sites,other problematic results of film defects include low film performancedue to poor junction formation and film delamination.

Provided herein are methods of laser polishing, texturing and cleaningthin-film photovoltaic materials. Laser polishing as described hereinrefers to smoothing a surface of a substrate or back contact layer byexposing one or more localized areas or sites of the surface to laserirradiation, thereby creating a melt pool centered on the site ofirradiation, and allowing the melted surface material to re-solidify. Incertain embodiments, high frequency surface roughness is removed. Incertain embodiments, surface waviness is removed.

Laser texturing as described herein refers to a process of exposing asurface of a substrate or back contact layer to laser irradiation tocreate a surface pattern having long range order. Long range orderrefers to a substantially uniform pattern over a surface area of atleast tens of square feet. In certain embodiments, laser texturinginvolves forming an interconnected low frequency patterned surface, thepattern surface frequency being between about 5 and 200 microns. Incertain embodiments, the surface is provided having textured featuresfrom a rolling or other process. Such a surface may be retextured usinglaser irradiation. In certain embodiments, a laser textured surface maybe characterized by one or more of: overlapping features, asubstantially uniform array of features having a center-to-centerdistance of between about 5 and 200 microns, a substantially uniformarray of features having circular symmetry, a substantially uniformarray of features having non-circular symmetry, or a substantiallyuniform array of features having a center-to-center distance less thanthe natural length scale of the features. Laser texturing may involverastering a laser beam in a first direction, while moving the surface ina second direction, such that features are formed across a surface at a100-1000 kHz repetition rate.

Laser cleaning as described herein refers to a process of removingorganic or inorganic contamination from a surface of a substrate or backcontact. The contamination may be in the form of discrete particles ormay be in the form of smeared material.

According to various embodiments, the methods involve removing defectsfrom a material surface by rapidly heating the surface in an area ofabout 5-200 microns to form shallow (about 10-1000 angstroms deep) andrapidly expanding melt pools, followed by rapid cooling of the surface.The minimization of surface tension causes the surface to re-solidify ina locally smooth surface. That is, high frequency roughness is smoothedout, with a lower frequency bump or texture pattern remaining in certainembodiments after resolidification. Cooling of the surface may be activeor passive according to various embodiments.

FIG. 1 includes a not-too-scale schematic of a thin-film photovoltaicstack 100 of a photovoltaic cell. Substrate 101 supports thin-filmlayers 103, 105 and 107. For the purposes of illustration, the figure isnot to scale; for example, thickness of the substrate 101 may be on theorder of mils, the thickness of the thin-film stack on the order ofmicrons (or hundredths of mils). Substrate 101 may be a metallic orpolymeric material. According to various embodiments, the substrate 101is relatively thin, such as for example, between about 2-100 mils, or incertain embodiments, 2-10 mils. However, other suitable thicknesses mayalso be used, e.g., 50 mils. Back electrical contact layer 103 provideselectrical contact to allow electrical current to flow through thephotovoltaic cell, and may be made of any appropriate material, e.g.,molybdenum, niobium, copper, silver, etc. Layer 107 is a top transparentelectrode layer. In certain embodiments, it is a transparent conductingoxide (TCO), for example, zinc oxide, aluminum-doped zinc oxide andindium tin oxide. Layer 105 represents several thin-films depositedbetween back electrical contact layer 103 and top transparent electricalcontact layer 107, including a p-n junction. To form the p-n junction, ap-type semiconductor layer is deposited on back electrical contact layer103 and an n-type semiconductor layer is deposited on the p-typesemiconductor layer. According to various embodiments, any suitablesemiconductor materials, such as CIGS, CIS, CdTe, CdS, ZnS, ZnO,amorphous silicon, polycrystalline silicon, etc. are used to form thep-n junction. For example, the p-type semiconductor layer may be CIGS orCIS, and the n-type semiconductor layer may be CdS or a cadmium freematerial, such as ZnS, ZnO, etc. Embodiments of the invention are notlimited to these p-n junctions, but include any appropriate p-n junctionproduced either as a homojunction in a single semiconductor material, oralternatively as a heterojunction between two different semiconductormaterials.

According to various embodiments, other thin-film layers may bedeposited between the electrical contact layers 103 and 107. As anexample, electrical contact layers 103 and 107 are separated by adistance of about 1-2 microns. Any ionic contamination and moisture,cutting defects (for non-monolithically integrated cells), conductivecontamination, or film delamination can lead to shunting sites,resulting in manufacturing or latent defects. In particular, thepresence of any defect in the substrate may cause shunting at the timeof deposition, or make the stack susceptible to developing shunts.

FIG. 1 also depicts a not-to-scale schematic illustration of twomonolithically integrated stacks at 102. In this example, a first stackincluding thin-film layers 103 a, 105 a and 107 a is electricallyconnected to a second stack including thin-film layers 103 b, 105 b and107 b at a site 111, with top contact layer 107 a and back contact 103 bseparated into interconnected sections. The site 111 of filmdiscontinuities is a likely point of moisture ingress, which can lead toshunting.

While there are various possible sources and causes of shunting andrelated defects, methods and apparatuses according to variousembodiments remove or reduce a significant number of such causes. Thephotovoltaic thin-film stacks depicted in FIG. 1 are deposited on asection of a substrate 101 after it has been polished, and contains nosubstantial defects, particles, or areas of high roughness. As such noshunting between the top contact layer 107 and metal contact layer 103and/or substrate 101 is observed.

FIGS. 2A-2C depict examples of laser polishing according to variousembodiments by directing a pulsed laser beam to one or more sites on asurface. First, in FIG. 2A, a substrate surface 201 is depicted. Thesubstrate may be a metal foil, such as a stainless steel foil, or apolymeric material. In certain embodiments, the substrate material is ahomogenous or non-homogenous composition of one or more metals,including steel, stainless steel, aluminum or titanium. In anotherembodiment, the substrate is a polyimide material. Substrate surface 201is a normal appearing material surface after mechanical processing suchas mechanical rolling, that is it contains high frequency roughness. Topolish the substrate, a single laser pulse is directed at the surface.The absorbed energy creates a local melted region on the surface. Thismelted region expands, and a wave begins to move outward from theirradiated spot. Because the substrate is highly conductive, the thermalenergy is lost quickly, resulting in a surface as depicted at 201′. Theresulting surface feature represents a close approximation to a constantvolume process. Because the material reaches a liquid state during thepolishing process, any high frequency roughness (as present in surface201) or other short wavelength features are lost due to the minimizationof surface energy during the transitory liquid phase. The resultantlower frequency (longer wavelength) roughness is appropriate for filmdeposition and photovoltaic applications. The slope of the raisedfeature in polished surface 201′ created by the laser pulse is much lessthan the slopes of the unpolished features in 201. FIG. 2B depicts asubstrate surface 201 coated with an as-deposited back contact surface203. In the example shown, the substrate surface is not polished priorto depositing the back contact layer, though in other embodiments, themethods involve laser polishing the substrate surface prior todeposition of the back contact layer in addition to laser polishing theback contact layer surface. As described above, the back contact may bemolybdenum, niobium, chromium, or others, or combinations of these. FIG.2B also shows the back contact surface 203′ after a laser pulsetreatment. In certain embodiments, the thickness of the back contact issufficient to maintain the laser process within the back contact. Forexample, depending on the laser pulse energy, a back contact layerthickness of about 1-5 microns is sufficient to prevent the laser energyfrom affecting the underlying substrate surface.

In addition to retexturing the surface, the laser polishing processcleans the surface in certain embodiments. This is depicted in FIG. 2C,which shows a particle 204 and an organic smear 206 on a surface 202.Surface 202 may be a substrate surface or a back contact surface. If a 1micron particle on a surface was left in place prior to coating thesurface with a 1 micron film, significant discontinuities would berequired to cover the particle. Particles larger than the overlyingthickness cannot be fully covered. Both situations result in potentialconductive paths to the back contact. In the methods according tocertain embodiments, a single laser pulse removes the surface materials,with the polished section of the surface 202′ free of them. Because theinitial part of the process involves a rapid influx of heat andexpansion of the surrounding surface, most surface materials are easilyremoved. It should be noted that for illustration, FIGS. 2A-2C arenot-to-scale with the vertical scale much larger than the horizontalscale, such that the actual slopes are much shallower than they appear.

Exposing the surface to a laser beam in accordance with the embodimentsdescribed herein may be performed in air, a controlled benignenvironment such as a noble gas (depending on the reactivity of thesurface metal to air), in a reactive gas environment for incorporatingelemental compositions into the surface material. In certainembodiments, the compositions or elements are placed on the surfaceprior to treatment for incorporation into the material during the meltphase. This is discussed further below.

In certain embodiments, a substrate is provided in a web and thin-filmsare deposited on the substrate in a roll-to-roll process. According tovarious embodiments, the substrate is laser polished prior to thedeposition of any films thereon. An example of a web path that includeslaser polishing is described below with respect to FIG. 3, which shows asimplified schematic diagram illustrating a top view of a laserpolishing and deposition apparatus. A web 301 of a bare substrate isshown being unspooled to enter the first deposition chamber 304. One ofskill in the art will understand that the manner in which the web is fedinto the chamber, the orientation of a spool holding the web, theorientation of the deposition chambers, etc., may be varied. Asdescribed below, in certain embodiments, the web is vertical ornear-vertical during laser polishing.

In certain embodiments, the web material is a thin metal foil such asstainless steel, though it may be a thin polymeric material. In manyembodiments, the web substrate has a width of between about 2 and 4 feet(in the direction perpendicular to the view), though the laser polishingapparatus may be configured to support substrates of any size, includingsizes outside this range.

In certain embodiments, the substrate is relatively low-cost, rolledsheet stock suitable for use as metallic or non-metallic substrates.These include stainless steel, aluminum, titanium, alloys of aluminum ortitanium, any metallic foil, or even a metalized non-metallic substrate.Examples of aluminum and titanium alloys include aluminum-silicon alloyand titanium-aluminum alloy, respectively; an example of a metalizednon-metallic substrate is a flexible, non-conductive substrate, such asa polymer substrate, with a sputtered metallic layer; and an example ofa stainless steel is 430-alloy stainless steel.

Prior to passing into the first deposition chamber 304, the depositionsurface of the substrate web is irradiated with one or more laser beamsat certain points on the surface at a laser polishing station 302. Thiscauses a melt pool to form at each of these points, evening out surfaceroughness as well as removing any particles that may be present on thematerial.

After polishing, the web passes into multiple deposition chambers 304,306 and 308, which contains one or more magnetrons and targets (notshown) for sputtering one or more thin-films on the web substrate. Forexample, molybdenum, copper or other back electrical contact materialmay be sputtered onto the substrate as it continuously moves through thedeposition chamber 304. Various guide rollers (not shown) may be used toguide the web substrate through the deposition chamber as well asthroughout the apparatus. In certain embodiments, a deposition chambermay include a coating drum (not shown) to support the web substratewhile it moves through the deposition chamber. In general, the web iskept in constant tension throughout the machine, with rollers used asnecessary to spread the web, provide web steering, provide web tensionfeedback to servo controllers, and to run the web in desired positions.The web then passes into the next deposition chamber 306 for furtherthin-film deposition. For example, a p-type CIGS layer may be depositedin this chamber. Next the web is transferred into deposition chamber 308for deposition of the top layer or layers of the photovoltaic stack. Forexample, a thin layer of n-type material to form and maintain the p-njunction in coordination with the previous layer may be depositedfollowed by deposition of a highly conductive and transparent aluminumdoped ZnO layer that completes the top electrode.

The deposition apparatus described above is an example of thin-filmdeposition apparatuses that may be used according to variousembodiments. The number of deposition chambers used may range from oneto as many as necessary to deposit the thin-film materials. The number,type and thickness of thin-films deposited in each chamber may also bevaried depending on system implementation. For example, each filmdescribed above with reference to FIG. 1 may be deposited in one or moredeposition chambers as appropriate. Likewise, a single depositionchamber may be used to deposit multiple types of films.

Still further, the active absorber stack deposited on the back contactlayer after any of the methods described herein may include silicon(e.g. for amorphous, micro-crystalline, polycrystalline and tandemsilicon cells), copper-indium-gallium-(selenium and/or sulfur), CdSe,CdTe, organic oligomers or polymers, layers of inorganic and organicmaterials, and whose final form may include micro-particles,nano-particles, or quantum dots.

While FIG. 3 provides an example of a laser polishing apparatus in thecontext of a thin-film deposition apparatus, one of skill in the artwill understand that other the methods and apparatuses may be practicedwith various modifications. For example, cut pieces of substrates may bepolished rather than a substrate on a web. Also, while in certainembodiments, laser polishing is performed between unspooling of the rolland entering the deposition chamber, in other embodiments, polishing mayoccur at a time and/or location remote from the deposition itself andmay involve spooling the polished substrate. In certain embodiments, theback contact is polished either in addition to or in lieu of thesubstrate being polished. This may occur within a deposition apparatus,between deposition apparatuses, etc.

The area of each irradiated site may be between about 5 and 200 micronsaccording to certain embodiments, for example, between about 5 and 100microns, 5 and 25 microns, 5 and 50 microns, 100 and 200 microns or 150and 200 microns. In many embodiments, each irradiated site is a singlegenerally circular spot, having a diameter equal to the beam diameter atthe surface. The beam area is defined by a lens that directs the beam onthe substrate; in certain embodiments, the lens is shaped to directellipsoidal or other shaped spots on the substrate.

In certain embodiments, the smoothed surface is characterized byinterconnected surface features, with the features having a frequency ofbetween about 1 and 200 microns, according to certain embodiments, forexample, between about 5 and 200 microns about 5 and 100 microns, 5 and25 microns, 5 and 50 microns, 100 and 200 microns or 150 and 200microns. These interconnected surface features may have a maximum aspectratio of no more than 1:1.

Pulse duration (full width half max peak power) may range from about0.10-100 nanoseconds, e.g., 0.1-50 nanoseconds, 0.1-10 nanoseconds, or1-10 nanoseconds. Peak pulse energy may range from about 1 microjoule tomillijoules. Pulse wavelength may range from about 0.4-10 microns. Theseparameters may depend in part on the material surface being treated, aswell as the desired textured surface that results.

In certain embodiments, methods are provided for texturing the surfaceof a substrate for a thin-film photovoltaic stack. In certainembodiments, methods are provided for texturing the surface of a backcontact layer of a thin-film photovoltaic stack. FIG. 4 shows examplesof high frequency roughness, low frequency roughness (waviness) andflatness scales. At 410, is an example of surface roughness in film 402.The images in FIG. 4 are not shown to scale with the vertical scaleexaggerated for illustrative purposes. Peaks 411 are present in the filmwith the peak-valley roughness being on the same order as the lengthscale, which ranges up to 10 microns. The aspect ratios (peak to valleyvertical distance) of the features (peaks 411) may be 1:1 or greater,e.g., 2:1, 3:1 or even higher. Further details of surfacecharacteristics of as-received metallic substrates are described in U.S.Patent Publication No. 20090229666, incorporated by reference herein.

At 420, a film 402 having a lower frequency roughness (waviness) withlength scales (distance between features) of greater than 10 microns.Here, the length scales are greater than the peak to valley roughness,in certain cases at least an order of magnitude greater than the peak tovalley roughness. At 403, a film 402 that is “flat” with length scalesoften exceeding a millimeter, is depicted.

According to various embodiments, the texturing methods described hereinmodify surfaces having high frequency, high aspect ratio (e.g., 1:1 orabove) features, retexturing them so that they exhibit only lowfrequency or flat features as described above. It should be noted thatunlike some technologies in which photolithography, multi-step CMPplanarization, fly height control and other functional requirementsdemand controlled long-wavelength flatness, for thin-film photovoltaics,waviness is tolerable. Only the high aspect ratio surface features aredisruptive to the film.

In certain embodiments, overlapping texture patterns are formed byrastering one or more laser beams to irradiate multiple spots across anentire surface or a portion thereof. FIG. 5 shows an example surfacehaving a pattern of overlapping features created by laser reflow. A planview is shown at 510, with features 512 overlapping in area 514. Across-sectional view is shown at 520. In the depicted example, the beamis rastered back and forth in a linear fashion to form rows of features.An off-set is applied between rows to ensure complete coverage. Tocreate overlap, the distance between irradiation points is smaller thanthe length scale of the resulting features. In the example depicted inFIG. 5, the features of the substrate pattern include overlappingcircular raised peripheries and a recessed center region. This is anexample of an overlapping surface that may be formed.

According to various embodiments, texturing a surface of a photovoltaicstack or a substrate therefore comprises creating features having aspectratios, or height to diameter ratios of less than about 1, less thanabout 0.5, less than about 0.1, less than about 0.05, less than about0.01, or less than about 0.005. FIG. 6 is an image showing an example ofa laser textured feature formed in a nickel surface. The feature has aheight of 150 Angstroms and is 5 microns in diameter, a height todiameter ratio of less than 0.005. Note the long wavelengthcharacteristics of the surface roughness, the continuity of the featureand the lack of local features in the region of the bump.

FIG. 7A shows two images of surface finishes of rolled or coined 300series stainless steel surface prior to laser treatment. The images are60×60 micron sized images, with a z-scale of 500 nm per division.Valleys are observed in both images (above left axis). FIG. 7B shows animage of a surface finish of a 300 series stainless steel surface afterlaser treatment. The z-scale resolution is increased to 250 nm perdivision for additional clarity of the surface. As compared to theimages in FIG. 7A, the high frequency peaks are removed and a muchgentler, more continuous surface results from laser treating. No valleysare observed after laser treating.

In certain embodiments, texturing the substrate or back contact surfaceresults in substantially uniform features. These features may havesubstantially circular symmetry as depicted in FIG. 5. In alternateembodiments, the final surface of the melt includes non-uniformitiesand/or non-circular symmetries. Pits, bumps, sombrero shapes andellipsoidal shapes can be formed under specific conditions of beam size,shape, power intensity, pulse length, lens shape (spherical,cylindrical, etc.) and the like. In certain embodiments, an ellipsoidalpattern is achieved using a cylindrical front lens. FIG. 8 shows twoexamples of texturing a surface of substrate 801. At 810, a rolledsubstrate 801 is depicted. Prior to treatment, the substrate surface asdirectional features due to rolling (or other surface preparation)direction, which is indicated. Ellipsoidal features are created by usingan appropriately shaped lens and/or relative motion between the beam andsubstrate in the direction of the major axis of the ellipse. At 820, asurface texture pattern having elongated laser features 812 in thelongitudinal direction (in the direction of the surface preparationfeatures) is depicted. At 830, a surface texture pattern havingelongated laser features 812 in the cross-direction is depicted.

In certain embodiments, the substrate and/or back contact is textured toproduce a surface roughness that enhances light trapping. In certainembodiments, peak to peak distance between features to is between about0.5 microns to about 1 micron, or about 0.5 microns to about 0.9microns. In certain embodiments, texturing the substrate and/or backcontact involves reducing peak height.

In certain embodiments, laser polishing and retexturing techniques asdescribed above are performed on a back contact layer. In certainembodiments, the back contact layer is molybdenum, niobium, chromium,copper, silver, or combinations of these or other suitable material forforming an ohmic contact. In particular embodiments, the back contactlayer includes molybdenum. The laser polishing may be performed on thesurface of the deposited back-contact, either exclusively or combinedwith a substrate polish, prior to the deposition of the remainingphotovoltaic material set. FIG. 9 provides a process flow chartillustrating certain operations in a method of laser polishing a backcontact layer according to certain embodiments. First, a substrate isprovided (901). The substrate may be provided in a web, e.g., a web ofstainless steel material, or may be provided as cut sheets. Thesubstrate may be a metallic foil or a conductive polymer material. Incertain embodiments, the substrate is series 300 or series 400 stainlesssteel. The as-provided material may be machine rolled or otherwisepre-treated or worked. At the dimensions of interest to photovoltaiccells, machine rolled material exhibits high frequency roughness asdescribed above with respect to FIG. 4. The substrate is optionallypolished (903). According to various embodiments, polishing involveslaser polishing as described above. Alternatively, it may involvemechanical polishing, e.g., with an abrasive grit in lieu of or inaddition to laser polishing. The substrate is then provided to adeposition chamber, for deposition of the back contact thin-film thereon(905). In many embodiments, deposition of the back contact involves aphysical vapor deposition or sputter process, though the methods are notso limited and encompass back contact films deposited by any suitablemethod.

According to various embodiments, the thickness of the back contact filmis between about 0 and 5 microns, e.g., 1 and 5 microns. In certainembodiments, the back contact film thickness is about 5 microns orgreater to allow suitable texturing and polishing depths. In certainembodiments, the back contact is less than 1 micron, e.g., about 0.5microns. In these cases, the depth of the melt pools may be controlledto be relatively shallow, e.g., 10-100 Angstroms.

Once the back contact film is deposited, laser polishing is performed(907). As described above, laser polishing involves exposing multiplelocalized areas or sites of the back contact film to a laser beam havingan intensity and for an exposure time sufficient to create a melt poolcentered on each of the localized sites. According to variousembodiments, the back contact is exposed to a laser beam to removeparticles and organic material that may be present on the film surfaceor within the film. Also, according to various embodiments, one or morelaser beams is rastered over the surface in a specific pattern totexture the surface as described above with respect to FIGS. 5-8.

In certain embodiments, polishing the as-deposited back contact layeralters the microstructure of the back contact layer from having acrystalline structure, or localized areas of crystalline structure, to asubstantially amorphous material. In certain embodiments, this involvesreducing the thickness of the layer. Also in certain embodiments,polishing the back contact enhances its diffusion barriercharacteristics. That is, after polishing, the back contact lowers adiffusion coefficient for the absorber stack constituents, as well asfor the constituents of the underlying substrate. In certainembodiments, at least the surface of the back contact is altered to bean amorphous material.

After the back contact film is polished and, in certain embodiments,retextured, an absorber layer is deposited on the back contact layer(909). In certain embodiments, adhesion of the CIGS or other absorberlayer is improved by the laser polishing. The adhesion is improved bythe presence of textured features and/or the presence of amorphous metalon the surface of the back contact.

Polishing the back contact may result in a superior surface fordeposition of an absorber stack than polishing only the underlyingsubstrate. For example, defects introduced during sputtering of the backcontact onto a polished or unpolished substrate are eliminated bypolishing the substrate.

Another aspect of the invention involves incorporating an element orcompound into a substrate or back contact surface during a lasertreatment process. In certain embodiments, sodium is incorporated into aphotovoltaic stack substrate and/or back contact during a laserpolishing, cleaning or retexturing treatment. In particular embodiments,sodium is incorporated into a substrate and/or back contact layer of aCIGS photovoltaic stack. Sodium diffusion has been found to beadvantageous during subsequent CIGS growth on the back contact as wellas improve conductivity in the CIGS layer.

FIGS. 10 and 11 are process flow charts illustrating operations inmethods of incorporating sodium into a CIGS photovoltaic stack accordingto various embodiments. First, in FIG. 10, a substrate for thin-filmdeposition is provided (1001). The substrate may be a metallic orconductive polymeric substrate. In certain embodiments, the substrate isan aluminum substrate or stainless steel. The substrate is then treatedwith sodium or a sodium containing compound (1003). Treating thesubstrate may involve exposing it to sodium vapor such that sodiumabsorbs onto the substrate, spraying, depositing or otherwise applying acoating of a metal salt to the substrate, applying a coating compoundcontaining sodium to the substrate, etc. In certain embodiments, a thinlayer of sodium or sodium containing compound, e.g., NaF, NaCl, NaS, isdeposited as a solid or precipitated as a liquid on the surface. Incertain embodiments, a thin layer of a compound of metal and sodium isdeposited on the surface of the substrate.

The sodium coated or treated substrate is then laser treated (1005).Laser treating involves exposing the substrate to radiation from a laserbeam at multiple spots across the substrate. This creates a melt pool ofthe sodium and substrate material at each spot, allowing incorporationof the sodium into the substrate surface. If a sodium-containingcompound is used, laser treating dissociates the compound, with thesodium intermixed in the surface layers. The remaining byproducts (e.g.,fluorine, chlorine, and the like) are reacted with a controlled reactivegas or getter system. In certain embodiments, operations 1003 and 1005overlap or are performed simultaneously. For example, in certainembodiments, the substrate is exposed to an overpressure of sodium vaporor a sodium containing compound as it is irradiated. This results in thesodium being intermixed in the surface layers.

The back contact layer, e.g., a molybdenum film, is then deposited onthe substrate (1007). An absorber layer, e.g., a CIGS layer, is thendeposited on the back contact (1009). The sodium diffuses from thesubstrate to the back contact, where it may diffuse from to reach theCIGS layer.

In certain embodiments, the methods involve forming a sodiumconcentration gradient in the substrate material, with the highestconcentration at the top or front surface. This is possible due to thelaser treatment of the substrate surface. Accordingly, in certainembodiments, the methods require less sodium. According to variousembodiments, sodium is incorporated to be between about 0.5% and 5% byweight of the substrate.

FIG. 11 is process flow chart illustrating operations in another methodof incorporating sodium into a CIGS stack in which the sodium isincorporated into the back contact layer in the same manners asdescribed above for the substrate. In this method, a substrate isprovided as described above (1101). The substrate may be polished orunpolished. A back contact, e.g., a molybdenum thin-film, is thendeposited on the substrate (1103). The back contact is then treated withsodium (1105). Treating the back contact may involve exposing it tosodium vapor such that sodium absorbs onto the back contact, spraying,depositing or otherwise applying a coating of a metal salt to thesubstrate, applying a coating compound containing sodium to thesubstrate, etc. In certain embodiments, a thin layer of sodium or sodiumcontaining compound, e.g., NaF, NaCl, NaS, is deposited as a solid orprecipitated as a liquid on the back contact surface. In certainembodiments, a thin layer of a compound of metal and sodium is depositedon the surface of the back contact

The back contact then undergoes laser treatment, to melt the molybdenumor back contact, and in certain cases sodium, thereby incorporating thesodium into the back contact (1107). In certain embodiments, operations1105 and 1107 overlap or are performed simultaneously. For example, incertain embodiments, the back contact is exposed to an overpressure of asodium vapor or a vapor of a sodium-containing compound as the backcontact is irradiated. An absorber layer, e.g., a CIGS layer, is thendeposited on the back contact (1109). The sodium diffuses from thesubstrate to the back contact, where it may diffuse from to reach theCIGS layer.

According to various embodiments, the sodium may be substantiallyuniformly incorporated in the back contact layer, or it may beincorporated such that there is a concentration gradient with thehighest concentration at the top surface of the back contact layer.Other methods of incorporating sodium into the back contact generallyinvolve the reverse concentration gradient, e.g., by supplying a sodiumsource such as lime glass at the back side of the metal contact.According to various embodiments, sodium is incorporated to be betweenabout 0.5% and 5% by weight of the back contact layer.

While the above examples refer to incorporation of sodium into thesubstrate and/or back contact layer, other elements or compounds may beincorporated in addition to or instead of sodium. These compounds may becoated on or otherwise applied as described above. In certainembodiments, a noble or reactive gas is used in conjunction with thelaser process to control the final material composition in thesolidified substrate or back contact melt. An example of an apparatus toimplement this aspect of the invention is described below with referenceto FIGS. 13A and 13B.

In a particular example, oxygen or an oxygen-containing gas is flowedover the surface being polished to increase oxygen content in thepolished film. In another example, a reducing gas is flowed over thesurface to reduce the oxygen content. For example, oxygen content may becontrolled to control the mobility characteristics of species diffusingthrough molybdenum.

Provided herein are apparatuses for carrying out the above-describedmethods. The methods may also be performed with other laser systems.FIGS. 12A and 12B depict an apparatus according to certain embodiments,with FIG. 12A showing a perspective view and FIG. 12B a side view of theapparatus. As described above, in certain embodiments, a web substrateis laser polished prior to entering a deposition apparatus. Theapparatus is configured to laser treat a web substrate, e.g., a thinmetal foil, and/or a back contact layer deposited on a web substrate.

In the figures, the web 1201 is tensioned between two guide rollers 1204a and 1204 b. Unpolished substrate is around roller 1204 a, polishedsubstrate around roller 1204 b. In certain embodiments, one or both ofthe guide rollers may not be present, e.g., they may be replaced with aspool. For example, in certain embodiments, unpolished substrate may betaken off a spool, polished and then guided into a deposition apparatusas described above with respect to FIG. 3.

A laser source 1203 emits a laser beam that is scanned in a cross-webdirection by three galvo system 1206. Individual beams 1214 are emittedfrom each galvo system 1206. The system, including web, laser source andgalvo mirrors are at a slight off-angle (mounting angle 1209) which mayin certain embodiments, 1°-20° from vertical, or 5°-15° from vertical toensure particulates and materials released from the laser process fallaway from the film, and do not fall onto the galvo lenses.

In certain embodiments, an air shower 1211 is used together with gravityand an ionization system 1213 to further ensure continued cleanoperation across the entire web and to ensure the expulsion of releasedmaterial. The galvo systems 1204 may be spaced about one foot or lessaway from the web, and include galvo mirrors to raster the beam in across-web direction. In certain embodiments in which multiple galvosystems are used, one or more of the galvo systems includes a beamsplitter to direct a beam to the next galvo system as well to the web.This allows, according to certain embodiments, a single laser source1203 to be used with beam splitters in the galvo systems 1206. Therastering process is in the near-vertical direction, while the web 1201transverses along the length direction. One or more backside heating orcooling elements 1207 is placed near or along the web to control thetemperature as needed. According to various embodiments, backsidetemperature is controlled at a set level between about 0 and 100° C.

Using multiple lasers, or a single laser with split beams, and multiplegalvo systems increase system throughput. The laser source may be any ofa single, double or quadrupled Nd:YAG or Nd:YLF laser, a carbon dioxidegas laser, and excimer source, or any other laser capable of producingthe power (10-1000 W), power density (5-200 micron spot size),controlled pulse duration (0.1-100 ns) and repetition rate (100 kHz-1000kHz) used for the treatment method. The laser wavelength may range fromabout 400 nm to 10 microns. The galvo systems accurately supportadjacent spot placement tolerances of less than 5 microns, or less than2 microns, at a repeating frequency of at least about 100 kHz. Inalternate embodiments, multiple laser sources are used, or a singlelaser source and galvo system may be sufficient depending on the websize.

FIGS. 13A and 13B illustrate a gas over-pressure system according tocertain embodiments, which may be used to incorporate sodium or othercomponents into a surface material. A split gas manifold 1315 ispositioned on either side of the laser beam 1314. The manifold 1315extends across the web to flow gas in the proximity of the laseroperation. A top view of the manifold 1315 is depicted in FIG. 13B. Themanifold is configured to direct gas toward intersection of the laserbeam 1314 and the web 1301 from either side of the laser beam 1314.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teaching. The embodimentsdescribed herein were chosen and described in order to best explain theprinciples of the invention and its practical application, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

1. A method of polishing a back contact of a thin-film photovoltaicstack, the method comprising: providing a substrate having a backcontact layer for a thin-film photovoltaic stack deposited thereon; andpolishing a front surface of the back contact layer by forming surfacefeatures in the front surface, wherein forming surface featurescomprises rastering one or more laser beams over the front surface. 2.The method of claim 1 wherein the back contact layer comprises amaterial selected from the group consisting of molybdenum, niobium,copper and silver.
 3. The method of claim 1 wherein the back contactlayer comprises molybdenum.
 4. The method of claim 1 wherein thesubstrate is a metal foil.
 5. The method of claim 1 wherein thesubstrate is a stainless steel foil.
 6. The method of claim 1 furthercomprising depositing one or more thin-film layers configured to absorbsolar energy on the back contact.
 7. The method of claim 6 wherein theone or more thin-film layers comprise CIGS or CIS.
 8. The method ofclaim 1 wherein rastering one or more laser beams over the front surfacecomprises increasing the amorphous character of the back contact layer.9. The method of claim 1 wherein the substrate is providing as a web andwherein polishing a front surface of the back contact layer comprisesforming multiple rows of surface features in the front surface, whereinforming a row of surface features comprises rastering one or more laserbeams over the front surface in a cross-web direction.
 10. The method ofclaim 9 wherein the surface features of a row overlap with the surfacefeatures of each adjacent row.
 11. The method of claim 1 wherein theback contact is about 0.5 microns thick.
 12. The method of claim 1wherein the back contact is between about 1 and 5 microns thick.
 13. Themethod of claim 1 wherein the surface of the web substrate is not laserpolished prior to deposition of the back contact layer.
 14. The methodof claim 1 further comprising laser polishing the surface of the websubstrate prior to the deposition of the back contact layer.
 15. Amethod of polishing a back contact of a thin-film photovoltaic stack,the method comprising: providing a substrate having a back contact layerfor a thin-film photovoltaic stack deposited thereon; irradiating afront surface of the back contact layer with laser energy to heat thesurface in localized areas of between 5 to 200 microns and form aplurality of shallow, rapidly expanding melt pools; after forming themelt pools, cooling the front surface to form a smoothed surface. 16.The method of claim 15 wherein the irradiating the front surface withlaser energy comprises exposing the front surface to laser beams at arate of about 10-1000 kHz.
 17. The method of claim 15 wherein thesmoothed surface comprises interconnected surface features, saidfeatures having a frequency of 1-200 microns.
 18. The method of claim 17wherein the interconnected surface features have a maximum aspect ratioof no more than 1:1.
 19. The method of claim 15 further comprisingcontrolling the temperature of a back surface of the substrate duringlaser irradiation of the front surface of the back contact.
 20. Themethod of claim 15 wherein the substrate is oriented at an angle of nomore than 10° from vertical during the irradiation.